Plasma display panels are widely used in applications such as gasoline dispensers. A conventional plasma display panel comprises a pair of glass panels which are sealed to form a chamber. The chamber contains selected ionizable gases, such as neon and/or argon, at low to sub-atmospheric pressures. Typically, anodes are located on the inside surface of the upper panel of the chamber and cathodes are located on the inside surface of the lower panel of the chamber. Producing an electric potential and current flow between a given anode and cathode causes the gas therebetween to ionize and glow. By strategically producing electric potentials between selected anode and cathode pairs, visible symbols may be displayed for viewing through the upper glass panel.
Mercury vapor is provided in the display chamber to inhibit sputtering. Sputter is a process in which the gas ions, propelled by the anode to cathode electrical potential, collide with and dislodge atoms from the cathode surface. These sputtered cathode atoms may deposit on the clear anode surface and build up a thick layer which will block the display's light output. Sputtering cathode atoms that become redeposited are often conductive (i.e. nickel cathodes), thus leading to electrical leakage paths or shorting inside the display. A third way that sputtering atoms can cause display failure is that the sputtered atoms, when they redeposit, may trap gas mixture atoms (i.e. gas cleanup). The reduction in pressure due to gas cleanup can further increase the rate of sputtering. Preferential cleanup of argon from a Penning Mixture can lead to a higher firing voltage. If the display's power supply cannot produce this higher voltage, the display will fail to light.
One widely recognized problem with plasma displays as described above involves condensation of the mercury vapor at low temperatures. In outdoor applications such as gasoline dispensers, ambient temperature can be as low as-30.degree. C. When the mercury condenses, there is a reduction in the amount of mercury vapor. Mercury condensation has other potential damaging effects which are well-known and will not be detailed herein.
Several display heater configurations have been developed to impede or prevent mercury condensation.
U.S. Pat. No. 4,956,573 to Smith et al. discloses a gas discharge display which includes a heater element that is built into the unit. The heater element is coplanar with the cathode electrode of the display. A single set of parallel conductive pins is used to provide the connection to the anode, cathode and heater element components of the display.
U.S. Pat. No. 4,520,290 to Cokefair discloses a gas discharge display in which a heater is built into the unit. A single set of parallel conductive pins provide the connections for the unit's anode, cathode and heater strip.
U.S. Pat. No. 4,730,139 to Harvey discloses a gas display panel having a base plate carrying cathode electrodes and a face plate carrying anode electrodes, the base plate and face plate being hermetically sealed together to form an envelope which is filled with an ionizable gas and mercury vapor. The base plate carries a conductor which is used to heat the panel and this conductor extends over the surface of the base plate into the areas of the seal between the base plate and face plate. The base plate also is positioned to provide heat at the location where a source of mercury is coupled to the base plate.
U.S. Pat. No. 4,692,655 to Person et al. discloses a gas discharge display device comprising upper and lower substrates having anodes and cathodes thereon. An envelope is formed between the upper and lower substrates and includes an ionizable gas therein. A resistance heater element is placed on the lower substrate adjacent the cathode, and a layer of dielectric material is printed over both the cathode and the heater. The heater includes a pair of trimming elements which extend parallel to one another and which may be connected at any one of a plurality of points along their length to achieve the desired trimmed resistance value for the heater element.
The heater configurations of the prior art have significant drawbacks. Gas plasma display heaters utilizing externally, rear-mounted heater elements and built-in heater elements have been found to be insufficient to adequately maintain the temperature throughout the display panels. This is because the glass that forms the panel is a poor heat conductor and therefore heat applied to the back of the rear panel is not efficiently transferred to the chamber and the front panel. The undesirable heat transfer profile is further compounded by the fact that, in gasoline dispenser applications, the front panel is exposed to significant convective and radiative cooling. The heated plasma display panels of the prior art which utilize built-in heating elements (that is, the elements are disposed between the glass panels) cannot be cost effectively retrofitted to existing plasma displays. Furthermore, on displays having the heater elements and the cathode on the same layer, the heater elements cannot be located over the entire backside of the display due to physical interferences. As a result of the aforementioned drawbacks of the prior art plasma display heaters, the extremities of plasma displays utilizing such heater configurations in extreme temperatures are often 50.degree. C. cooler than portions that are near the heating elements.
Thus, there exists a need for a heater apparatus for plasma displays which is effective to maintain a substantially uniform temperature across a plasma display. Further, there exists a need for such a heater apparatus which is cost effective to manufacture and mount on a plasma display. In addition, there exists a need for a such a heater apparatus which can be cost effectively retrofitted to existing plasma displays. Also there exists a need for a heater apparatus having the above characteristics which works passively.